The present invention relates to a process for preparing phase stabilized ammonium nitrate. Phase stabilized ammonium nitrate is particularly useful in a gas generating composition for inflating a vehicle occupant protection device and any other low-smoke gas generating or propulsion applications.
An inflator for inflating an inflatable vehicle occupant protection device, such as an air bag, contains an ignitable gas generating composition. The inflator includes an igniter. The igniter is actuated so as to ignite the gas generating composition when the vehicle experiences a collision for which inflation of the air bag is desired to help protect a vehicle occupant. As the gas generating composition burns, it generates a volume of inflation gas. The inflation gas is directed into the air bag to inflate the air bag. When the air bag is inflated, it expands into the vehicle occupant compartment and helps to protect the vehicle occupant.
Ammonium nitrate is particularly useful in gas generating compositions for inflating an inflatable vehicle occupant protection device because ammonium nitrate produces upon combustion high gas outputs and low levels of residual solids. A problem with the use of pure ammonium nitrate is that pure ammonium nitrate undergoes a series of phase transitions over the typical operating temperature range of an inflator (i.e., xe2x88x9240xc2x0 C. to about 107xc2x0 C.). In pure ammonium nitrate, phase transitions are observed at xe2x88x9218xc2x0 C., 32.3xc2x0 C., and 84.2xc2x0 C. The phase transitions are accompanied by structural and volumetric changes. Generally any structural and volumetric change is detrimental, and it is desired to limit any structural and volumetric change as much as possible.
Ammonium nitrate can be phase stabilized to reduce structural and volumetric changes associated with the phase transitions. Typical methods of phase stabilizing ammonium nitrate produce a phase stabilized ammonium nitrate product with a water content of about 0.10% by weight of the phase stabilized ammonium nitrate. Phase stabilized ammonium nitrate with a water content of 0.10% by weight can potentially degrade during the service life of the inflator.
The present invention is a process for preparing phase stabilized ammonium nitrate. The process includes preparing a solution of ammonium nitrate, a phase stabilizer, and an inert liquid. The solution is atomized to form a stream of droplets. The droplets are freeze dried to form phase stabilized ammonium nitrate.